1. Field of the Invention
This invention relates to an anisotropic rare earth magnet material having structural and magnetic anisotropy and also to a process for the production of same.
2. Description of the Prior Art
There are numerous intermetallic compounds of the rare earth metal(R)-Co type or the rare earth metal(R)-Fe type, including RM.sub.13, R.sub.2 M.sub.17, RM.sub.5, R.sub.5 M.sub.19, R.sub.2 M.sub.7, RM.sub.3, RM.sub.2, R.sub.2 M.sub.3, R.sub.2 M.sub.1.7, R.sub.4 M.sub.3, R.sub.24 M.sub.11, R.sub.9 M.sub.4, R.sub.3 M, etc., wherein M is Co or Fe. Their crystalline structures are diverse and include the cubic, hexagonal, orthorhombic and rhombohedral systems.
Rare earth magnets are magnet materials which contain both hexagonal and rhombohedral crystals, make use of R.sub.2 M.sub.17, RM.sub.5, R.sub.2 M.sub.7, RM.sub.3 and the like among intermetallic compounds in many instances, and have large crystalline and magnetic anisotropy. Compared with conventional Ba-ferrite magnets or Sr-ferrite magnets, Alnico-5, -6, -8 and -9 magnets, two-phase separated magnets of Fe-Cr-Co, Pt-Co magnets and Mn-Al-C magnets, their magnetic characteristics including those of both the RM.sub.5 and R.sub.2 M.sub.17 systems are significantly higher in coercive force IHc and maximum energy product (BH)max. The consumption of rare earth magnets has hence increased. Rare earth magnets have contributed considerably toward the size reduction of new industrial products and the improvements of their characteristics.
As the usual production processes of RM.sub.5 and R.sub.2 M.sub.17 which are produced currently, there may be mentioned primarily the sintering process and the polymer bonding process which makes use of a binder. According to the sintering process, raw materials are converted into roughly-ground powder by melting the raw materials or reducing oxidized powder directly. In order to protect rare earth metals which are active and susceptible to oxidation, the powder is finely ground into fine powder (5-20 .mu.m) in an organic solvent such as silicone oil. After drying, the fine powder is press-formed along an axis of easy magnetization such as C-axis in a magnetic field of 10-30 kOe (800-2400 kA/m), followed by sintering, solid solution treatment and aging in a nonoxidizing atmosphere to obtain structural and magnetic anisotropy. In addition to the above-described magnetic field pressing method, the unidirectional solidification method in which a magnet material is formed from a melt by the Bridgeman method, high-frequency zone melting method or the like may also be mentioned as a method for orienting crystals in one direction.
The polymer bonding process was developed to overcome the drawback of brittleness of sintered magnets. A prismatic portion which has grown in one direction from a melt is ground. The thus-formed powder is then hardened thermally or chemically with a resin binder, a rubbery and thermoplastic binder, respectively.
A description will next be made of the mechanism of magnetization of a sintered magnet. In the above-described rare earth magnets, the mechanism of their coercive force generation may be discussed separately under two different situations. One of the situations arises when the coercive force is relatively low, i.e., about 6 kOe (480 kA/m). Under this situation, a cellular micro-structure is formed with a rhombohedral cell of R.sub.2 M.sub.17 phase surrounded by boundaries of hexagonal crystals of RM.sub.5. When the coercive force is large, namely, higher than 14 kOe [&gt;14 kOe (&gt;1120 kA/m)], the cell size increases and a thin lamellar layer of RM.sub.3 occurs in a manner superposed in the cell structure. Namely, the reversal of magnetization takes place when new domain walls come out in crystal grains and then grow. Domain walls in the stage of their growth can move easily within their respective crystal grains. The above-mentioned cell boundaries however resist the movement of the domain walls, thereby exhibiting strong pinning action against the domain walls. The coercive force is determined by the size ( 500-2000 .ANG.) of the cell. The coercive force is generally said to increase as the cell size becomes greater.
Magnetism as high as 30-35 MGOe (240-280 kJ/m.sup.3) in terms of maximum energy product (BH)max is obtained at room temperature by such a mechanism of magnetization as described above. However, rare earth magnets are theoretically considered to have the capability to achieve (BH)max as large as 50-70 MGOe (400-560 kJ/m.sup.3). As a matter of fact, Dy.sub.3 Al.sub.2 has been found to show a coercive force of 20 kOe (1600 kA/m), namely, (BH)max of 73 MGOe (584 kJ/m.sup.3) at a low temperature (the temperature of liquefied helium). The mechanism of this magnetization is believed to be different from the conventional theory. Judging also from the fact that this phenomenon is very remarkable at low temperatures but the magnetism is reduced due to thermal agitation when the temperature is high, the pinning action against domain walls is not attributed solely to cell boundaries as has been observed in the conventional mechanism of magnetization but may also be attributed to stacking fault, Guinier-Preston (G.P.) zone and phase boundaries and also to dislocation, etc. Owing to this pinning action, the magnetism to be produced seems to reach a value close to the theoretical value and the mechanism of its magnetization is believed to differ from the present theory of magnetism.
Although the above-described sintering process has such merits that it is excellent in mass productivity, features high yield and can easily meet the requirement for various shapes, the sintering process tends to cause oxidation of powder being processed because the powder is fine, it is difficult to control oxygen in the grinding, drying and heat treatment steps. Since the powder is pressed and then sintered, it is technically difficult to obtain thin products of 1 mm and less due to slipping upon pressing, cracking upon sintering and the brittleness inherent to metal compounds. In addition, the powder oriented along the C-axis upon its sintering grows in random directions. The anisotropy is therefore prone to disturbance and the magnetism is deteriorated accordingly.
On the other hand, the polymer bonding process is accompanied by drawbacks that the magnets are thermally unstable (maximum service temperature: 200.degree. C.) and compared with sintered magnets, have lower densities and lower magnetic characteristics.